It is desirable to reduce the clearance between the tip of a rotary fan blade and the fan casing of a gas turbine engine in order to maximise fuel efficiency.
However, reducing the clearance between the fan blade tip and the fan casing can lead to undesirable rubbing of the fan blade tips on the fan casing.
It is known to provide a track liner on the interior surface of the fan casing to provide a shroud for the fan blade tips. The track liner is formed of an abradable material (e.g. an epoxy resin) which is abraded by the rotary fan blades to form channels in the track liner into which the fan blade tips extend. These abradable track liners have found some success with metallic fan blades (e.g. titanium blades) which are very durable but a disadvantage of these track liners is that the running clearance is set by the longest blade. These track liners are not appropriate for use with fan blades formed of composite material (e.g. fibre-reinforced plastic (or resin matrix) material) which are less durable and readily suffer blade tip damage.
It is known from US2013/0195633 to provide a rotary blade tip insert which is fitted into a slot in an aerofoil blade where the tip insert is formed of a material having a greater durability that the blade. For example, the blade is formed of aluminium and the tip insert is formed of anodized aluminium, titanium and/or ceramic. The tip insert is retained within the slot by mechanical mating, material attachment e.g. welding or by adhesive bonding. Once secured within the slot, the tip insert cannot move and thus cannot accommodate manufacturing/operational tolerances or extreme events such as bird strikes. Furthermore, the intimate contact between the tip insert and the slot means that heat (resulting from rubbing friction) and mechanical stresses are readily transferred from the tip insert to the blade.
There is a need for a tip insert that can accommodate manufacturing/operational tolerances and extreme events and/or which can reduce thermal/mechanical stress transfer from the tip insert to the fan blade.